Undoubtedly, the need for high spectral resolution (in particular the capability to resolve the Fe-L complex) is absolutely crucial to provide accurate constraints on chemical abundances (and the physics of the enrichment) in groups and elliptical galaxies. While waiting for the exploitation of micro-calorimeters onboard XRISM and Athena, one should keep in mind the valuable potential of the Reflection Grating Spectrometer (RGS) instrument onboard XMM-Newton to deliver high resolution spectra. Formally, the RGS has a spectral resolution of \(\sim \) 3 eV. In the case of extended sources, however, the slit-less characteristic of this instrument induces a convolution of a given line profile with the spatial distribution of its surface brightness along the dispersion direction of the detector. Although this makes the RGS abundance measurements of extended clusters rather challenging [1]}, groups are by definition more compact and are less affected by this instrumental broadening. The spectral window of the instrument (typically \(\sim \) 6–30 \(\mathring{A}\) , corresponding to \(\sim \)  0.4–2 keV) is both an advantage – as it covers the O VIII (and O VII), N VII, and even C VI lines which are difficult or impossible to detect with CCD instrumental responses – and a drawback – as the continuum is challenging to constrain within this band. Consequently, the power of RGS resides less in the measurements of the IGrM absolute abundances than in the measurements of their (relative) N/Fe, O/Fe, Ne/Fe, and Mg/Fe ratios. Although discrepancies of absolute Fe measurements between RGS and CCD-like instruments may in principle lead to discrepancies in their respective X/Fe ratios, we note a generally good agreement in the latter case [2]}.
